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=============================================================================
FreeBSD-SA-19:22.mbuf                                       Security Advisory
                                                          The FreeBSD Project

Topic:          IPv6 remote Denial-of-Service

Category:       kernel
Module:         net
Announced:      2019-08-20
Credits:        Clement Lecigne
Affects:        All supported versions of FreeBSD.
Corrected:      2019-08-10 00:01:25 UTC (stable/12, 12.0-STABLE)
                2019-08-20 17:49:33 UTC (releng/12.0, 12.0-RELEASE-p10)
                2019-08-10 00:02:45 UTC (stable/11, 11.3-STABLE)
                2019-08-20 17:49:33 UTC (releng/11.3, 11.3-RELEASE-p3)
                2019-08-20 17:49:33 UTC (releng/11.2, 11.2-RELEASE-p14)
CVE Name:       CVE-2019-5611

For general information regarding FreeBSD Security Advisories, including
descriptions of the fields above, security branches, and the following
sections, please visit <URL:https://security.FreeBSD.org/>.

I.   Background

mbufs are a unit of memory management mostly used in the kernel for network
packets and socket buffers.  m_pulldown(9) is a function to arrange the data
in a chain of mbufs.

II.  Problem Description

Due do a missing check in the code of m_pulldown(9) data returned may not be
contiguous as requested by the caller.

III. Impact

Extra checks in the IPv6 code catch the error condition and trigger a kernel
panic leading to a remote DoS (denial-of-service) attack with certain
Ethernet interfaces.  At this point it is unknown if any other than the IPv6
code paths can trigger a similar condition.

IV.  Workaround

For the currently known attack vector systems with IPv6 not enabled are not
vulnerable.

On systems with IPv6 active, IPv6 fragmentation may be disabled, or
a firewall can be used to filter out packets with certain or excessive
amounts of extension headers in a first fragment.  These rules may be
dependent on the operational needs of each site.

V.   Solution

Upgrade your vulnerable system to a supported FreeBSD stable or
release / security branch (releng) dated after the correction date,
and reboot.

1) To update your vulnerable system via a binary patch:

Systems running a RELEASE version of FreeBSD on the i386 or amd64
platforms can be updated via the freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install
# shutdown -r +10min "Rebooting for security update"

2) To update your vulnerable system via a source code patch:

The following patches have been verified to apply to the applicable
FreeBSD release branches.

a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.

# fetch https://security.FreeBSD.org/patches/SA-19:22/mbuf.patch
# fetch https://security.FreeBSD.org/patches/SA-19:22/mbuf.patch.asc
# gpg --verify mbuf.patch.asc

b) Apply the patch.  Execute the following commands as root:

# cd /usr/src
# patch < /path/to/patch

c) Recompile your kernel as described in
<URL:https://www.FreeBSD.org/handbook/kernelconfig.html> and reboot the
system.

VI.  Correction details

The following list contains the correction revision numbers for each
affected branch.

Branch/path                                                      Revision
- -------------------------------------------------------------------------
stable/12/                                                        r350828
releng/12.0/                                                      r351259
stable/11/                                                        r350829
releng/11.3/                                                      r351259
releng/11.2/                                                      r351259
- -------------------------------------------------------------------------

To see which files were modified by a particular revision, run the
following command, replacing NNNNNN with the revision number, on a
machine with Subversion installed:

# svn diff -cNNNNNN --summarize svn://svn.freebsd.org/base

Or visit the following URL, replacing NNNNNN with the revision number:

<URL:https://svnweb.freebsd.org/base?view=revision&revision=NNNNNN>

VII. References

<URL:https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=238787>
<URL:https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2019-5611>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-19:22.mbuf.asc>
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